ABSTRACT ! Recent advances to decode the genetics underlying inflammatory bowel disease (IBD) have led to the discovery of numerous IBD-associated genetic variants in the human genome. Our work studying these variants using genomics, mouse models, systems biology, and human patient samples has revealed the central role of autophagy and intracellular antimicrobial defense in the pathogenesis of IBD. In particular, characterization of IBD-associated I231L variant in the G protein-coupled receptor GPR65 revealed a key role of GPR65 as an environmental sensor that maintains lysosomal pH and function. Regulatory mechanisms that maintain lysosome homeostasis are critical in phagocytes such as macrophages, which require a highly developed lysosome network for delivery of microbicidal machinery to the phagosome. Recently, we have found that GPR65 interacts with the transcription factor, TFEB, that controls lysosome biogenesis. Here, we investigate the mechanisms by which GPR65 acts as a sensor, interpreting environmental cues and eliciting adaptive responses to maintain lysosomal homeostasis through TFEB. In Aim 1, we examine the role of GPR65 in controlling lysosome homeostasis through signaling intermediates and TFEB transcriptional activity. In Aim 2, we investigate the mechanisms by which impaired GPR65 signaling, and particularly the GPR65 I231L variant, results in colitis and pathogen susceptibility. In Aim 3, we will define how endogenous metabolites regulate TFEB activity. These research aims will advance our understanding of how metabolic sensors and lysosomal homeostasis mediate intracellular pathogen defense and could reveal a widespread role for lysosome biology in innate immunity.